An increasing shortage of primary care providers (PCP) is projected as the US population grows and ages. One approach to address the shortage is to increase the role of nurse practitioners (NP) as PCPs. Our preliminary data suggest a rapid growth in the percent of older patients who received outpatient care from NPs, from 0.4% in 1997 to 8.4% in 2008. By 2008, 2.5% of Medicare beneficiaries received the majority of their primary care from an NP. The rate of NP care varies substantially by geographic location. In 10 Hospital Referral Regions (HRRs), more than 10% of the elderly received the majority of their primary care from NPs, while less than 1% of the elderly received the majority of their care from NPs in 70 HRRs. NP practice is quite sensitive to state regulations. Compared with states with the most restrictive regulations, the odds of receiving the majority of primary care from NPs was three fold higher for beneficiaries in states with the most liberal regulations. Four of the five RCTs comparing care provided by NPs vs. physicians were conducted outside the US in different health care delivery systems, and the US study was from an academic health center. No national or population-based studies have yet assessed the processes, outcomes and costs of NP care. Our Specific Aims are: 1. Compare process and quality of care provided by NPs and generalist physicians, with a focus on processes of primary and secondary prevention, and intensity and frequency of visits, potential inappropriate use of medication, adherence to key medications, and receipt of specialist consultations. 2. Compare outcomes of care between care provided by NPs and generalist physicians, focusing on hospitalizations for ambulatory care sensitive conditions. 3. Compare Medicare costs between care provided by NPs and generalist physicians. To address these aims, we will use claims from national samples of Medicare beneficiaries from 2006 through 2011 to conduct this comparative effectiveness research.